1. Technical Field
The present invention relates to a reducing agent for low temperature reducing and sintering of copper nanoparticles and a method for low temperature sintering using the same.
2. Description of Related Art
Industrial inkjet printing technology has a number of advantages in economy of energy, low cost, multilayer lamination, formation of thin films and so on since it only uses material where it is needed with little or no waste and allows formation of finer patterns. Manufacturing products using inkjet printing needs technologies in preparation of particles for ink, combination of inks, inkjet head design to inject desired amount of ink, system to inkjet droplets where it is needed, printing to form fine wires, surface treatment, sintering and the like. Researches on materials suitable for inkjet printing have been expanded diversely to metals, ceramics, polymers, bio- and hybrid-inks, etc.
In methods for forming fine wiring patterns using metal nano-ink, methods using gold nanoparticles or silver nanoparticles have been developed.
In case of using gold nanoparticles, gold itself is costly, lending to high manufacturing unit cost of gold nanoparticles dispersion so that it presents a limitation to mass production in terms of economy.
Silver has been the most widely researched for the formation of inkjet printed metal wirings. Silver nanoparticle ink allows easy formation of metal wirings by steps of printing and heat treating in air and simultaneously provides electrical characteristics. However, silver nanoparticles are inefficient for high material price and cause disconnection (or opening) for electro-migration when width or space between wires becomes narrow.
Due to such problems of high material cost and disconnection of wires for electro-migration, copper has attracted great interest and there have been recently great researches on using copper. However, a disadvantage of copper nanoparticles is their tendency to oxidize easily compared to gold or silver which does not show oxidation problem.
Therefore, since copper nanoparticle ink is easily oxidized when it gets contact with ambient air during the sintering process, the sintering process requires essentially reducing atmosphere and the sintering temperature should be lower than the glass transition temperature of a polymer substrate to apply it to the polymer substrate.
A general method is a method to conduct the sintering process under the reducing atmosphere using hydrogen gas. However, the sintering is allowed only when it is heated at 250° C. or higher for a long period of time and the specific resistance is very high with this method. Thus, it is not practical since the glass transition temperatures of most of polymer substrates are 250° C. or lower.
Therefore, there is a large demand to develop low temperature reducing and sintering methods of copper nanoparticles which allow the sintering of copper nanoparticles at 250° C. or lower and result a low specific resistance.